A Gravitational Millilensing Candidate in GRB 111012A with a Possible Compact Merger Origin

Abstract We identify GRB 111012A in the Fermi Gamma-ray Burst Monitor archive as a candidate case of gravitational millilensing. The burst shows two pulses separated by ∼3.1 s, reminiscent of previously reported lensed events such as GRB 950830 and GRB 200716C. The temporal profiles of the two pulses are highly consistent: autocorrelation analysis indicates similarity at a significance level above 4 σ , and a chi-squared comparison supports the hypothesis that they arise from the same underlying distribution. Additionally, their spectra are consistent within 3 σ uncertainties. Although the overall duration of ∼8 s is longer than typical short bursts, the spectral lag and minimum variability timescales are consistent with those of short gamma-ray bursts. Interpreting the delay and flux ratio as a lensing signature implies a redshifted lensing mass 1.27( ± 0.12) × 10 5 M ⊙ , consistent with an intermediate-mass black hole. However, the discrepancy in the hardness ratio might be caused by the structured jet in gamma-ray bursts (GRBs) or the central engine’s memory effect. GRB 111012A adds to the rare population of candidate lensed GRBs, although no confirmed case of a gravitationally lensed GRB currently exists.